Lipase-mediated selective acetylation of primary alcohols in ethyl acetate

Article abstract of DOI:10.1016/j.tetlet.2017.12.041

An environmental friendly process to selectively acetylate primary alcohols was demonstrated. The esterification process consists of treatment of a primary alcohol in the presence of immobilized C. antarctica lipase (Novozyme-435) in ethyl acetate at room temperature. Primary alcohols were acetylated in the presence of secondary alcohols and phenols.

Full text of DOI:10.1016/j.tetlet.2017.12.041

Accepted Manuscript
Lipase-mediated selective acetylation of primary alcohols in ethyl acetate
Ernane C. de Souza, Moises Romero-Ortega, Horacio F. Olivo
PII:
S0040-4039(17)31544-7
https://doi.org/10.1016/j.tetlet.2017.12.041
TETL 49543
DOI:
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
13 November 2017
2 December 2017
11 December 2017
Please cite this article as: de Souza, E.C., Romero-Ortega, M., Olivo, H.F., Lipase-mediated selective acetylation
of primary alcohols in ethyl acetate, Tetrahedron Letters (2017), doi: https://doi.org/10.1016/j.tetlet.2017.12.041
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Lipase-mediated selective acetylation of
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Ernane C. de Souza, Moises Romero-Ortega, and Horacio F. Olivo

1
Tetrahedron Letters
Lipase-mediated selective acetylation of primary alcohols in ethyl acetate
a
a,b
a,
Ernane C. de Souza, Moises Romero-Ortega, and Horacio F. Olivo 
a
Medicinal and Natural Products Chemistry, The University of Iowa, Iowa City, IA 52242, USA
Departamento de Química Organica, Facultad de Química, Universidad Autónoma del Estado de México, Toluca, México.
b
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
An environmental friendly process to selectively acetylate primary alcohols was demonstrated.
The esterification process consists of treatment of a primary alcohol in the presence of
immobilized C. antarctica lipase (Novozyme-435) in ethyl acetate at room temperature. Primary
alcohols were acetylated in the presence of secondary alcohols and phenols.
Available online
2
009 Elsevier Ltd. All rights reserved.
Keywords:
Acetylation
Ethyl acetate
Lipase
Chemoselective
1
.
Introduction
environmentally friendly, practical, safe, inexpensive, and
performed under mild conditions. In most cases, the product does
not require chromatographic purification. The reaction is filtered
thru a small bed of celite or filter paper to recover the catalyst,
and solvent is evaporated.
Lipases (triacylglycerol ester hydrolases) are versatile enzymes
largely employed in synthetic organic chemistry to resolve
racemic mixtures of alcohols, leading to highly enantioselective
products. Furthermore, lipases are promiscuous biocatalysts
for several recently found applications such as the aldol reaction,
the carbon-carbon Michael addition to -unsaturated carbonyl
compounds, the aza-Michael additions of amines to acrylates,
1-5
1
2
vinyl acetate,
H
OH
OAc
OH
H
OH
OAc
OH
lipase
H
OH
OAc
OH
H
OAc
+
+
+
+
+
+
3
4
6
Cl
H
H
H
Cl
H
H
H
2
CH Cl
Cl
H
H
H
Cl
H
H
H
2
5
the Mannich reaction, and the Ugi multicomponent reaction.
Lipases are also used in the amidation of acids and esters.
7
8
H
H
lipase
H
H
OH
In principle, the lipase-mediated esterification/hydrolysis of
Cl
Cl
acetone-water
Cl
Cl
9
alcohol/ester reactions are reversible. In order to drive the
acylation reaction to the products, vinyl acetate is commonly
used as the acylating reagent, Scheme 1. Thus, the ene alcohol
produced is irreversibly tautomerized to acetaldehyde. On the
other hand, when a hydrolysis reaction is carried out for the
resolution of esters, the reaction is performed in water, which
acts as a nucleophile and also as solvent. Therefore, the reaction
equilibrium is displaced towards the alcohol. We have previously
employed ethyl acetate, an environmentally benign solvent, in
lipase-mediated reactions in the presence of urea-hydrogen
peroxide. Ethyl acetate readily reacts in the binding site of the
lipase with a serine residue forming an acetyl-enzyme complex.
Hydrogen peroxide reacts with the acetyl-enzyme complex
lipase
OAc
H
H
H
H
Cl
Cl
ethyl acetate
Cl
Cl
Scheme 1. Lipase-mediated resolution of alcohols.
There are several chemical methods available to carry out the
acylation of alcohols. Most of them include the use of acyl
halides or anhydrides in the presence of bases such as pyridine or
10
13
14
triethylamine, Lewis or protonic acids, as well as solid acid
15
11
catalysts. Vinyl acetate can also be used as the acetylating agent
in the presence of palladium catalyst. Acyl halides and acetic
16
anhydride are lachrymator and corrosive agents, respectively, and
less than ideal for industrial applications. The present protocol
for acetylating alcohols utilizing ethyl acetate in the presence of
an immobilized lipase under mild reaction conditions should
complement well other lipase-mediated acylation methods and be
forming
in
situ
acetylperoxycarboxylic
acid.
The
peroxycarboxylic acid can then oxidize alkenes to oxiranes and
also insert oxygen in cyclic ketones (Baeyer-Villiger oxidation)
to deliver lactones. In this paper, we describe the use of ethyl
acetate as the solvent as well as the acetyl-transfer agent for the
acetylation of primary alcohols.
—
17
considered a cleaner alternative.
1
2
This reaction is
——

Corresponding author. Tel.: +1-319-335-8845; fax: +1-319-335-8766; e-mail: horacio-olivo@uiowa.edu

2
Tetrahedron Letters
2.
Results and Discussion
Interestingly, no acetylation was observed for secondary alcohols
3-pentanol, cyclopentanol, cyclohexanol and 1,4-
(
Initially, the acetylation of benzyl alcohol (1a) in ethyl acetate
was investigated employing a small amount of lipase, Table 1. C.
antarctica lipase (recombinant enzyme expressed in Aspergillus
niger and immobilized in acrylic resin, Novozyme-435) is a
popular serine-lipase that has shown to accept a large variety of
alcohols in its binding site. The lipase-mediated acetylation was
carried out at different concentrations of ethyl acetate at room
temperature for up to 24 hours (entries 1-6). Conversion was
cyclohexanediol) under the general reaction conditions even
after seven days (not shown on the table). These secondary
alcohols can be acylated when the lipase-mediated reaction is
21
carried out with vinyl acetate as the acylating agent.
18
Table 2. Lipase-mediated acetylation of primary alcohols in
ethyl acetate.
1
determined by H NMR signal integration of the benzylic
hydrogens of the alcohol and ester. The NMR signals for the
benzylic hydrogens of the alcohol appear at 4.70 ppm, while the
upfield signals for the same hydrogens of the ester appear at 5.15
ppm. No significant conversion difference was observed varying
the amount of ethyl acetate. A maximum conversion was
achieved after 24 h (entries 6 and 7).
a
Table 1. Acetylation of benzyl alcohol.
Novozyme-435
ethyl acetate
OH
OAc
1a
2a
Entry
EtOAc (mL)
Time (h)
16
Conv. (%)
1
2
3
4
5
6
7
1
2
3
4
5
5
5
77
80
85
83
79
93
93
16
16
16
16
24
48
a
The reactions were performed with 100 mg of benzyl alcohol and 5
mg of Novozyme-435.
The trans-chlorocyclopropyl methanol (1b) is an important
intermediate for the synthesis of the macrocyclic marine lactone
19
callipeltoside, Scheme 2. This compound is easily prepared in
racemic form. The lipase-mediated acetylation of
chlorocyclopropyl methanol in ethyl acetate was completed after
two days (100% conversion). When the reaction was stopped at
50% conversion, the alcohol and the ester were separated by
column chromatography. The unreacted alcohol was converted to
2
0
its Mosher ester. Enantiomeric excess of the alcohol was
determined by integration of the methylene hydrogens of both
1
diastereomers by H NMR. No difference in enantioselectivity
was found when running the reaction under classical conditions.
lipase
OH
OAc
H
OH
H
OH
H
H
+
+
Cl
H
Cl
H
ethyl acetate
Cl
H
Cl
H
2
1
days, 100% conv.
day, 50% conv.
(
+)-1b
(-)-1b
(+)-1b, 45% ee
2b
Scheme 2. Lipase resolution of trans-chlorocyclopropylmethanol.
We expanded this lipase-mediated acetylation to several
aliphatic alcohols, Table 2. Acetylation of long-chain
hydrocarbon alcohols, octanol and lauryl alcohol reached very
good and high conversions after one day (entries 1 and 2). It took
one day for 1,4-butanediol and 1,5-pentanediol to reach 94% and
a
Conditions: Novozyme-435 (5 mg), ethyl acetate (5 mL) and 100
mg of the correspondent alcohol. Reaction was stirred in a test tube
with a magnetic bar at room temperature.
92% conversions, respectively (entries 3 and 4). 4-Methyl-
pentanol 1g reached very high conversion after one day and N,N-
dimethylaminopropanol 1h did too after two days (entries 5 and
We also investigated the lipase acetylation of aromatic alcohols
in ethyl acetate, Table 3. The 2-methylbenzylic alcohol was more
difficult to acetylate than the unsubstituted benzylic alcohol
(entry 1). The p-methoxybenzylic alcohol was completely
acetylated in two days (entry 2), while the 3,5-
ditrifluoromethylbenzylic alcohol took three days to reach a 94%
conversion (entry 3). The three phenolbenzylic alcohols were
acetylated exclusively on the benzylic hydroxyl group after three
days (entries 4-6). Longer chain on the primary alcohol
possessing an aromatic ring favored the acetylation (entries 7-9).
Also, benzylic biphenols were good substrates for acetylation
6
). Acetylation of aminoethoxyethanol 1i occurred both on the
amino and the hydroxy group reaching completion after two days
entry 7). Acetylation of 2-methyl-butanol 1j reached almost
(
completion after two days and one day for the more lipophilic
cyclohexylmethanol 1k (entries 8 and 9). However, the more
hindered and hydrophilic 2,2-dimethylpropanediol 1l only
reached 70% conversion after seven days (entry 10). Both trans
and cis-geraniols were acetylated respectively in two and three
days in 75 and 73% conversions (entries 11 and 12).

3
after three days (entries 10 and 11). In general, acetylation of
aromatic alcohols took longer than the ones of aliphatic alcohols.
3. Conclusions
In summary, we have reported herein a green, simple, efficient
and selective method for acetylating primary alcohols over
secondary, tertiary and phenolic hydroxyl functionalities
employing an immobilized lipase (Novozyme-435) as catalyst
and ethyl acetate as both solvent and acetyl-transfer agent under
mild conditions. No chromatographic purification was necessary
in most cases. Furthermore, the cheapness and availability of the
reagents, easy work-up, minimal generation of waste, side
Table 3. Lipase-mediated acetylation of aromatic alcohols in
ethyl acetate.
products or hazardous compounds, make this
a useful
environmentally benign process to be added to the present
methodologies for the acquirement of selective acetylated
primary alcohols.
Acknowledgments
The authors are grateful to CAPES (Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior), CONACYT
(Consejo Nacional de Ciencia y Tecnologia) and the University
of Iowa for financial support.
a
Conditions: Novozyme-435 (5 mg), ethyl acetate (5 mL) and 100 mg of the
correspondent alcohol. Reaction was stirred with a magnetic bar in a test tube
at room temperature.
(
d) de Souza, R. O. M. A.; Matos, L. M. C.; Goncalves, K. M.;
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9
2k
1
1
(
4
1
0
2l
7
2
Supplementary Material
Supplementary materials related to this article can be found at
http://dx.doi.org/
11
2m
1
2
2n
3
Highlights
a
Conditions: Novozyme-435 (5 mg), ethyl acetate (5
mL) and 100 mg of the correspondent alcohol. Reaction
was stirred in a test tube with a magnetic bar at room
temperature.
Lipase-mediated selective acetylation of
primary alcohols in ethyl acetate

Use of ethyl acetate as an acetyl source for the
acetylation of alcohols



Catalytic use of Novozyme-435
Selective acetylation of primary alcohols
Table 3. Lipase-mediated acetylation of aromatic
alcohols in ethyl acetate.
Reaction is stirred at room temperature, filtered and
evaporated
a
entry
acetate (2)
time
(
d)
Table 2. Lipase-mediated acetylation of aliphatic
alcohols in ethyl acetate.
a
1
2o
2p
2
entry
acetate (2)
time conv.
(
d)
(%)
1
2
2c
1
85
2
1
2d
1
1
95
94
3
2e

5
3
2q
3
94
4
5
6
2r
2s
2t
3
3
1
83
94
97
7
8
2u
2v
1
1
77
94
9
2w
2x
1
1
98
97
1
0
1
1
2z
1
79
a
Conditions: Novozyme-435 (5 mg), ethyl acetate (5
mL) and 100 mg of the correspondent alcohol. Reaction
was stirred in a test tube with a magnetic bar at room
temperature.